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03/14/2000 08:00:00

Caltech grad student's team first to detect radio emission from a brown dwarf

A graduate student in astronomy from the California Institute of Technology recently led a team of researchers in finding the first radio emission ever detected from a brown dwarf, an enigmatic object that is neither star nor planet, but something in between.

The discovery, reported in the March 15 issue of the journal Nature by lead author Edo Berger and his colleagues, demonstrates that brown dwarfs can flare 10,000 times more intensely than theory predicted. The results will likely force experts to rethink their theories about magnetism in brown dwarfs and gas giants, says Berger's supervisor, Shri Kulkarni, who is John E. and Catherine T. MacArthur Professor of Astronomy and Planetary Science at Caltech.

Berger was leader of a student team that made the discovery during a special National Science Foundation student summer program at the NSF's Very Large Array (VLA) near Socorro, New Mexico. The brown dwarf they observed is named LP944-20.

Berger and his colleagues decided to make a long-shot gamble in attempting to observe a brown dwarf from which X-ray flares had been recently discovered with NASA's Chandra X-ray satellite.

"We did some background reading and realized that, based on predictions, the brown dwarf would be undetectable with the VLA," said Berger. "But we decided to try it anyway."

After consulting with Dale Frail, an astronomer at the National Radio Astronomy Observatory (NRAO), Berger and his colleagues decided to utilize a block of observing time traditionally dedicated to the summer students.

The day after they collected their data, the students gathered at the NRAO array operations center in Socorro to process the data and make the images. Berger, who had prior experience processing VLA data, worked alone in the same room as the other students, who were working together on another computer. Berger finished first and was shocked at his image.

I saw a bright object at the exact position of the brown dwarf, and was pretty sure I had made a mistake," Berger said.

He waited for the others, who were working under the guidance of another NRAO astronomer. Ten minutes later, the others also produced an image on the screen in which the same bright object showed up at the brown dwarf's location.

Berger then began breaking up the approximately 90 minutes' worth of data into smaller segments. His results showed that the brown dwarf's radio emission had risen to a strong peak, then weakened. This demonstrated that the brown dwarf had flared.

"Then we got real excited," Berger said, adding that the students immediately sought and received additional observing time. Soon they had captured two more flares.

"The radio emission these students discovered coming from this brown dwarf is 10,000 times stronger than anyone expected," Frail said. "This is going to open up a whole new area of research for the VLA."

The existence of brown dwarfs—objects with masses intermediate between stars and planets—had long been suspected but never confirmed until 1995, when Kulkarni made the first observation at Caltech's Palomar Observatory. Since then, a large number of brown dwarfs have been identified in systematic surveys of the sky. Astronomers now believe that there are as many brown dwarfs as stars in our galaxy.

Flaring and quiescent radio emissions have been seen previously from stars and from the giant planets of our solar system, but never before from a brown dwarf. Moreover, the strength of the magnetic field near the brown dwarf—as inferred from the radio observations—is well below that of Jupiter and orders of magnitude below that of low-mass stars, said Kulkarni.

Conventional wisdom would require large magnetic fields to accelerate the energetic particles responsible for the radio emissions. The same conventional wisdom says that brown dwarfs are expected to generate only short-lived magnetic fields.

However, the persistence of the radio emission of LP944-20 shows that the picture is not complete, Kulkarni said.

"I am very pleased that a first-year Caltech graduate student was able to spearhead such an undertaking, which led to this big discovery," said Kulkarni. "This discovery will spur theorists into obtaining a better understanding of magnetism in stars and planets."

In addition to Berger and Frail, the other authors of the paper are Steven Ball of New Mexico Institute of Mining and Technology, Kate Becker of Oberlin University, Melanie Clark of Carleton College, Therese Fukuda of the University of Denver, Ian Hoffman of the University of New Mexico, Richard Mellon of Penn State, Emmanuel Momjian of the University of Kentucky, Michael Murphy of Amherst College, Stacy Teng of the University of Maryland, Timothy Woodruff of Southwestern University, Ashley Zauderer of Agnes Scott College, and Bob Zavala of New Mexico State University.

[Editors: Additional information on this discovery is available at the NRAO Web site at http://www.nrao.edu/pr/browndwarf.html]